246 



COMPARATIVE ANATOMY 



would be difficult to recognise them as Gastropods, or even as Molluscs, were it not 

 proved that the larvae of one of these forms at least are distinctlj' Gastropodan 

 larvse. The incomplete state of our knowledge of the development of these two 

 parasites, and- the absence of any transition forms between them and the typical 

 organisation, make them very difficult to understand. 



Entocolax Ludwigii inhabits endoparasitically the body cavity of a Holothxirian 

 (Myriotrochus Ilinkii), one end of its vermiform body being attached to the body 

 wall of its host. Its organisation, a scheme of which is given in Fig. 205, can be 

 best studied with the help of some hypothetical transition forms, through Avhich a 



A 



B 



Fio. 204.— A, B, C, D, Hypothetical transition stages between Thyca and Stilifer on;tli© 

 one side and Entocolax (Fig. 20.f.) on the other (after Schiemenz). u, Aiuls ; fd, pedal gland ; 

 Z, liver (digestive gland); Id, hepatic iiite.stine ; in, moutli ; mag, .stomach; o, ovary; o/, aperture of 

 the false mantle ; sf, false foot ; mn, false mantle ; ?(, uterus ; «', body wall of the host. 



Gastropod of the type of Thyca or Stilifer might pass in developing into an 

 endoparasitic parasite like Entocolax. Fig. 204 A shows the first stage, which still 

 much resembles Thyca, and is still ectoparasitic ; Fig. 204 B, C, D are further 

 stages in development. In proportion as the animal becomes endoparasitic, and 

 gives up its relations to the external world, do the sensOry organs, the shell, and the 

 mantle cavity with the gill disappear. The stomach, as a separate section of the 

 intestine, degenerates, the digestive gland (liver) becomes a simple unbranched 

 diverticulum of the intestine, which loses the rectum and anus. All organs for the 

 purpose of mastication at the anterior end of the alimentary canal are lost. The 



